UNITED NATIONS — The UN nuclear chief said Monday that Iran is not cooperating with an investigation into suspected secret work on nuclear weapons.

Yukio Amano told the UN General Assembly that talks between the Vienna-based International Atomic Energy Agency and Iran have intensified this year after
the agency said in November 2011 it had ‘‘credible information that Iran had carried out activities relevant to the development of a nuclear explosive device.’’

‘‘However, no concrete results have been achieved so far’’ in the talks, Amano said.

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While the atomic energy agency continues to verify that Iran’s declared nuclear material is not being diverted from peaceful purposes, ‘‘Iran is not providing the necessary cooperation to enable us to provide credible assurance about the absence of undeclared nuclear material and activities,’’ Amano said.

‘‘Therefore, we cannot conclude that all nuclear material in Iran is in peaceful activities,’’ he said. But the agency’s director general said ‘‘the agency is firmly committed to intensifying dialogue with Iran.’’

‘‘We will continue negotiations with Iran on a structured approach,’’ he said. ‘‘I hope we can reach agreement without further delay.’’

Iran has repeatedly denied any interest in possessing nuclear arms, but the international community fears that Tehran may turn its peaceful uranium enrichment program toward weapons making — a concern that is growing as the government expands the number of machines it uses to enrich its stockpile of enriched uranium. As those fears grow, so does concern that Israel could carry out its threats to attack Iran’s nuclear facilities before that nation reaches the bomb-making threshold.